The present invention relates generally to brake control apparatus, and more particularly to such a brake control apparatus suitably employed for a motor vehicle with a diagonal braking hydraulic pressure supply system.
As one of braking hydraulic pressure supply systems for braking wheels of a front-engine and front-driven motor vehicle and the like is known a diagonal braking hydraulic pressure supply system, which is arranged such that, for example, a braking hydraulic pressure generated in a master cylinder is applied through a first hydraulic pressure supply passage to wheel braking cylinders for the front-left and rear-right wheels and further applied through a second hydraulic pressure supply passage to wheel braking cylinders for the front-right and rear-left wheels. That is, in FIG. 1 showing such a hydraulic pressure supply system for a two-channel (2ch) type anti-skid system, a hydraulic pressure produced by a master cylinder 2 in response to depression of a braking pedal of the motor vehicle is supplied through a hydraulic pressure supply passage 100 to a wheel braking cylinder 3a for the front-right wheel FR and a wheel braking cylinder 3d for the rear-left wheel RL and further supplied through another hydraulic pressure supply passage 200 to a wheel braking cylinder 3b for the front-left wheel FL and a wheel braking cylinder 3c for the rear-right wheel RR. In each of the hydraulic pressure supply passages 100 and 200 is provided a hydraulic pressure control device 1a or 1b for allowing execution of the two-channel anti-skid control, for example. In addition, at the rear-wheel side position of each of the hydraulic pressure supply passages 100 and 200 is provided a proportioning valve (P valve) 4a or 4b.
One problem in such a conventional braking arrangement relates to deterioration of the travelling stability and increase in braking distance, because the P valves 4a and 4b are of the characteristic-fixed type. That is, assuming that the P valve has a characteristic as indicated by a solid-line X in FIG. 2, the characteristic of the P valve is generally determined and designed so that the hydraulic pressure distribution between the front and rear wheels becomes adequate when the motor vehicle is running on the most ordinary, dry asphalt road surface, thereby allowing deterioration of the vehicle travelling stability and the braking efficiency in accordance with the states of the road surface and the tires. For example, in the case that a front-wheel driven vehicle runs on a snow-piled road, a winter-type tire such as spike tire is employed for the front wheel and a normal tire is employed for the rear wheel. In this case, the hydraulic pressure in the wheel braking cylinder for the front-wheel which is subjected to a greater friction with respect to the road surface is required to be more increased in order to shorten the braking distance of the motor vehicle.
That is, in FIG. 2, under the condition that an adequate ratio of the hydraulic pressures for the front and rear wheels is obtained at a point B whereby the locking of the front and rear wheels can be prevented, according to a P valve, in the case that the hydraulic pressure corresponding to the point B is applied to the front wheel in order to shorten the braking distance, the hydraulic pressure corresponding to a point C is applied to the rear wheel to thereby result in deterioration of the travelling stability. On the other hand, when the hydraulic pressure corresponding to the point B is applied for the rear wheel, the hydraulic pressure indicated by a point A is applied for the front wheel to thereby result in lack of the braking force and excessive braking distance. Further, assuming that the motor vehicle is running on a friction-different road having different frictional coefficients for the front and rear wheels, it is preferable that the hydraulic pressure of the high-friction side wheel braking cylinder is controlled to become higher and the hydraulic pressure of the low-friction side wheel cylinder is set to be lower. However, this is difficult in the case of the 2ch anti-skid apparatus for the diagonal hydraulic pressure supply system. That is, when the right side of the motor vehicle is the high-friction side and the left-side thereof is the low-friction side, although it is preferable that, in the FR-RL system, the hydraulic pressure in the wheel braking cylinder for the front-right wheel FR is set to be relatively high and the hydraulic pressure in the wheel cylinder for the rear-left wheel RL is set to be relatively low, due to the above-mentioned characteristic of the conventional P valve, the braking force for the rear wheel becomes excessive or the braking force for the front wheel becomes insufficient. On the other hand, although it is preferable that, in the FL-RR system, the hydraulic pressure in the wheel braking cylinder for the front-left wheel is set to be relatively low and the hydraulic pressure in the wheel braking cylinder for the rear-right wheel is set to be relatively high (point D in FIG. 2), due to the P valve characteristic, the braking force for the rear-right wheel becomes insufficient to results in lengthening the braking distance when the the wheel braking pressure for the front-left wheel is set to be appropriate, whereas the braking force for the front-left wheel becomes excessive to result in locking tendency when the wheel braking cylinder pressure for the rear-right wheel is set to be adequate (point E in FIG. 2).
Thus, in the conventional 2ch type anti-skid control apparatus employed for the diagonal hydraulic pressure supply system, difficulty can be encountered to ensure the travelling stability and further shorten the braking distance under all conditions in which the motor vehicle is placed.